Thermal Air Flow Sensor

ABSTRACT

A thermal air flow sensor that produces less measurement error is provided. The thermal air flow sensor includes: a semiconductor substrate; a heating resistor, resistance temperature detectors, and an electrical insulator that includes a silicon oxide film, wherein the heating resistor, the resistance temperature detectors, and the electrical insulator are formed on the semiconductor substrate; and a diaphragm portion formed by removing a portion of the semiconductor substrate. The heating resistor and the resistance temperature detectors are formed on the diaphragm portion. The thermal air flow sensor further includes a silicon nitride film formed as the electrical insulator above the heating resistor and the resistance temperature detectors. The silicon nitride film has steps conforming to the patterns of the heating resistor and the resistance temperature detectors. The silicon nitride film has a multilayer structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/355,104, filed Apr. 29, 2014, which is a 371 of InternationalApplication No. PCT/JP2011/006592, filed Nov. 28, 2011, the disclosuresof which are expressly incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a thermal air flow sensor, ameasurement device used for air flow meters, that measures air flow witha heating resistor and a resistance temperature detector.

BACKGROUND ART

The thermal air flow meter that can directly detect air volume hasbecome the mainstream air flowmeter. In particular, a thermal air flowmeter provided with a measurement device produced by semiconductormicromachining has attracted interest as a low-cost,low-power-consumption air flow meter. For example, PTL 1 proposes ameasurement device (thermal air flow sensor) for use in such thermal airflow meters. The thermal air flow sensor proposed by this publicationincludes an electrically insulating film formed on a semiconductorsubstrate, heating resistors and resistance temperature detectors formedon the electrically insulating film, and an electrical insulator formedon the heating resistors and the resistance temperature detectors. Theheating resistor and resistance temperature detector region has adiaphragm structure formed by removing a portion of the semiconductorsubstrate from the back side by anisotropic etching.

CITATION LIST Patent Document

PTL 1: JP-A-2010-133897

SUMMARY OF INVENTION Technical Problem

In the thermal air flow sensor proposed by PTL 1, the heating resistorand resistance temperature detector region has a diaphragm structure,and a silicon oxide film, a silicon nitride film, and a silicon oxidefilm are laminated on the surfaces of these resistors by using a plasmaCVD (chemical vapor deposition) method. Films formed by CVD aretypically coarse (low atom density), and are subjected to ahigh-temperature (1,000° C.) heat treatment to densify the films. Thesilicon nitride film generates a particularly high stress during thisheat treatment.

The heating resistor and the resistance temperature detector are formedby deposition and patterning of a metal film such as a molybdenum film,and the surfaces of the silicon oxide film and the silicon nitride filmdeposited on these surfaces have steps conforming to the thickness ofthe metal film. The high stress in the silicon nitride film concentratesin these step portions, and may cause cracking in the film. The cracksallow entry of oxygen and moisture from the surface, and cause theresistors to oxidize. The oxidation varies the resistance of theresistors as it progresses, and produces measurement errors in the airflow meter.

It is an object of the invention to provide a thermal air flow sensorthat produces less measurement error.

Solution to Problem

In order to achieve the foregoing object, a thermal air flow sensor ofthe invention includes a semiconductor substrate; a heating resistor,resistance temperature detectors, and an electrical insulator thatincludes a silicon oxide film, wherein the heating resistor, theresistance temperature detectors, and the electrical insulator areformed on the semiconductor substrate; and a diaphragm portion formed byremoving a portion of the semiconductor substrate, the heating resistorand the resistance temperature detectors being formed on the diaphragmportion, and the thermal air flow sensor further comprising a siliconnitride film formed as the electrical insulator above the heatingresistor and the resistance temperature detectors, wherein the siliconnitride film has steps conforming to the patterns of the heatingresistor and the resistance temperature detectors, and wherein thesilicon nitride film has a multilayer structure.

Advantage Effects of Invention

With the invention, a thermal air flow sensor can be provided thatproduces less measurement error.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic plan view of a measurement device of FirstEmbodiment of the present application.

FIG. 2 is a cross sectional view of First Embodiment according to thepresent application.

FIG. 3 is a diagram representing a distribution of the generated stressin a silicon nitride film.

FIG. 4 is a diagram showing that the generated stress in a siliconnitride film is dependent on silicon nitride film thickness.

FIG. 5 is a photographic representation of interface formation after thedeposition of a second silicon nitride film on a first silicon nitridefilm.

FIG. 6 is a cross sectional view of Second Embodiment of the presentapplication.

FIG. 7 is a diagram representing the dependence of steps on thegenerated stress in a silicon nitride film.

FIG. 8 is a diagram representing how the steps of a silicon oxide filmare reduced by mechanical polishing and etch back.

FIG. 9 is a diagram representing hillock formation on heating resistorsand resistance temperature detectors.

DESCRIPTION OF EMBODIMENTS

Embodiments of the invention are described below.

Embodiment 1

A thermal air flow sensor as First Embodiment of the invention isdescribed below with reference to FIGS. 1 and 2. FIG. 1 is a schematicplan view of the thermal air flow sensor. FIG. 2 is a cross sectionalview taken at A-A′ of FIG. 1.

As illustrated in FIG. 1, the thermal air flow sensor (a measurementdevice used for a thermal air flow meter) of the present embodiment isconfigured to include a silicon substrate 1, heating resistors 10,resistance temperature detectors 9 and 11 for measuring air temperature,terminal electrodes 12, and a diaphragm portion 6. The diaphragm portion6 has an end portion 8.

A producing method of the present embodiment is described below withreference to FIG. 2.

The silicon substrate 1 is thermally oxidized to form a thermallyoxidized film 2 that becomes a lower electrically insulating film. Amolybdenum (Mo) film is then deposited on the thermally oxidized film 2in a thickness of about 150 nm, and the heating resistors 10 and theresistance temperature detectors 9 and 11 are formed by patterning. Thelower electrically insulating film may be formed solely by the thermallyoxidized film 2, or may be formed as a laminate with a silicon nitride(SiN) film or a silicon oxide film (SiO₂). The structure heatingresistors 10, and the resistance temperature detectors 9 and 11 may beformed by using a metal film, such as platinum, instead of using themolybdenum film. Thereafter, a silicon oxide film 3 (upper electricallyinsulating film) is deposited on the heating resistors 10 and theresistance temperature detectors 9 and 11 in a thickness of about 500 nmby using a plasma CVD method. This is followed by a heat treatment at atemperature of 800° C. or more to densify the film. The heat treatmentforms steps 13 corresponding to the thickness of the depositedmolybdenum. A silicon nitride film 4 is then formed in a thickness ofabout 200 nm by a two-step deposition procedure. Here, a heat treatmentis always performed after each deposition, specifically after the firstdeposition and the second deposition of the silicon nitride film. Theheat treatments are performed at 800° C. or more, a temperature thatallows the silicon nitride film to be densified. Because the siliconnitride film 4 is deposited on the steps 13 of the silicon oxide film 3,the silicon nitride film 4 formed has steps 14. Thereafter, a siliconoxide film 5 is deposited in a thickness of 300 to 500 nm by using aplasma CVD method, and a heat treatment is performed at 800° C. or more.This is followed by deposition and patterning of a polyimide-based resinfilm to form a PIQ film 15. Referring to FIG. 1, the terminal electrodes12 are formed by depositing aluminum, gold, or the like through contactholes formed through the upper electrically insulating film afterforming the silicon oxide film 5 (not illustrated). Finally, thediaphragm portion 6 is formed from the back surface, using a siliconoxide film or the like as masking material, and an etchant such as KOH(not illustrated). The diaphragm portion 6 may be formed by using a dryetching method. Referring to FIG. 2, the etching mask (masking material)has an end portion 7. The masking material covers the area outside theetching mask end portion 7, and etching removes the silicon substratematerial in the region of the diaphragm portion 6.

In the present embodiment, the upper electrically insulating film has athree-layer structure configured from the silicon oxide film 3, thesilicon nitride film 4, and the silicon oxide film 5. However, the upperelectrically insulating film may be configured from more than threelayers.

Advantages of the present embodiment are described below.

The present inventors conducted studies to find the cause of thecracking the occurs when the silicon nitride film 4 is formed as amonolayer film, and found that cracks are caused by a high tensilestress, as high as 1,000 MPa, that generates inside the film, andconcentrates on the surfaces of the steps 14 as a result of the siliconnitride film 4 being densified by the high-temperature heat treatmentperformed after the deposition of the silicon nitride film 4.

In the thermal air flow sensor, the heating resistors 10 generate heatof about 200 to 300° C. at all times, and the sensitivity of the airflow sensor increases as the heating temperature increases. The heat ofthe heating resistors 10 heats the thermally oxidized film 2, thesilicon oxide film 3, the silicon nitride film 4, and the silicon oxidefilm 5 disposed in the vicinity. The film stress of these films varieswith the temperature of heat treatment. Thus, absent a high-temperatureheat treatment, the heat of the heating resistors 10 varies the filmstress, and deforms the diaphragm. This causes air flow detection errorsas the resistance values of the heating resistors 10 and the resistancetemperature detectors 9 and 11 vary because of the piezoresistiveeffect. In the present embodiment, this is prevented by the heattreatment of 800° C. or more, which is always performed after thedeposition of the upper and lower electrically insulating films.

In common LSIs, aluminum is used for the wiring material, andelectrically insulating films such as a silicon oxide film and a siliconnitride film are formed after the deposition of the aluminum film.Because the melting point of aluminum is about 550° C., the depositionof the silicon oxide film and the silicon nitride film on the aluminumis not followed by high-temperature annealing of 500° C. or more, andthe silicon nitride film does not generate high tensile stress. Further,the heat-induced film stress changes hardly occur because of the lowheat generation, at most 125° C., of the LSI. Further, because the LSIdoes not have a diaphragm structure, there is hardly any substratebending due to film stress changes, and the electrical properties of theLSI remain unaffected. The cracking problem of the silicon nitride film4 is indeed specific to thermal air flow sensors.

FIG. 3 represents the result of the analysis of the generated stress inthe silicon nitride film 4. It can be seen that the stress isconcentrated in the surface of the silicon nitride film 4. FIG. 4 showsthat the stress is dependent on the thickness of the silicon nitridefilm 4. The stress decreases with decrease in thickness. It is temptingto think from this result that the stress can be effectively reduced bymaking the silicon nitride film 4 thinner. However, reducing thethickness of the silicon nitride film 4 lowers the diaphragm strengthagainst dust collisions, and it is not desirable to reduce the thicknessof the silicon nitride film 4.

The stress concentrated in surface portions of the silicon nitride film4, as shown in FIG. 3. The cause of the generated stress in the siliconnitride film 4 was the film contraction due to heat treatment. Lessstress generated when the silicon nitride film 4 was made thinner.Reducing the film thickness lowers stress because the thinner thicknessinvolves less film contraction. It is therefore considered possible tolower the surface stress by providing the silicon nitride film 4 as amultilayer film, and reducing the film thickness of the last layer.Specifically, a heat treatment is performed after the deposition of thefirst layer to contract the film, and a heat treatment is also performedafter the second deposition. In this way, the surface stress generatedin the silicon nitride film 4 only comes from the contraction of thesecond layer film, and the generated stress can be reduced withoutvarying the film thickness.

This method involves interface formation between the first layer and thesecond layer, as shown in FIG. 5. As to the thickness ratio of the firstand second layers in the two-layer structure, the effect remains evenwhen the second layer is thicker than the first layer, because thesecond layer will still be thinner than the total thickness. However,the stress can be reduced more effectively when the second layer isthinner than the first layer.

The silicon nitride film 4, described as having a two-layer structure inthe foregoing First Embodiment, may have a three- or four-layerstructure.

In any case, the surface stress generated in the silicon nitride film 4can be effectively reduced when the films in the multilayer structurehave the same thickness, or when the film formed last is the thinnest ofthe other films in the multilayer structure.

Embodiment 2

Second Embodiment differs from First Embodiment only in the producingmethod, and will be described with respect to the producing method, withreference to FIG. 6(a) to (c).

As shown in FIG. 6(a), the silicon substrate 1 is thermally oxidized toform a thermally oxidized film 2 that becomes a lower electricallyinsulating film. A molybdenum (Mo) film is then deposited on thethermally oxidized film 2 in a thickness of 150 nm, and the heatingresistors 10 and the resistance temperature detectors 9 and 11 areformed by patterning. The lower electrically insulating film may beformed solely by the thermally oxidized film 2, or may be formed as alaminate with a silicon nitride (SiN) film or a silicon oxide film(SiO₂). The heating resistors 10, and the resistance temperaturedetectors 9 and 11 may be formed by using a metal film, such asplatinum, instead of using the molybdenum film. Thereafter, a siliconoxide film 3 (upper electrically insulating film) is deposited on theheating resistors 10 and the resistance temperature detectors 9 and 11in a thickness of about 600 nm to 700 nm by using a plasma CVD method.This is followed by a heat treatment at a temperature of 800° C. or moreto densify the film. The heat treatment forms steps 13 corresponding tothe thickness. The steps 14 are then planarized by mechanical polishing(CMP) to make the thickness of the silicon oxide film 3 about 500 nm, asshown in FIG. 6(b). As used herein, “planarize” means eliminating stepsthat conform to the patterns of the heating resistors 10 and theresistance temperature detectors 9 and 11. A silicon nitride film 4 isthen deposited in a thickness of about 200 nm, and a heat treatment isperformed at 800° C. or more to densify the silicon nitride film 4, asshown in FIG. 6(c). Thereafter, a silicon oxide film 5 is deposited in athickness of 300 to 500 nm by using a plasma CVD method, and a heattreatment is performed at 800° C. or more. The terminal electrodes 12,and the diaphragm portion 6 are then formed in the same manner as inEmbodiment 1.

Advantages of the present embodiment are described below.

The cause of the cracking that occurs in the silicon nitride film 4 isthe high film stress, and the stress concentration due to the stepformation. In Embodiment 1, this is counteracted by reducing the filmstress. In Embodiment 2, the steps 14 are removed by mechanicalpolishing to suppress stress concentration and the stress-induced crackgeneration.

The common planarization procedures used in LSI wiring layer formationis intended to eliminate the contact failure of the contact wiresconnecting the lower layer wiring and the upper layer wiring. This is incontrast to the present embodiment in which the planarization preventsthe crack generation in the electrically insulating film. That is, thepurpose of the planarization is different.

The stress can be reduced by reducing the steps as shown in FIG. 7,instead of completely planarizing the silicon oxide film 3 in the manneras shown in FIG. 6(b). Because the steps can be reduced by usingmechanical polishing or etch back, the generated stress can be reducedto suppress crack generation. Needless to say, the amount of etching isless than the thickness of the heating resistors 10 and the resistancetemperature detectors 9 and 11. Further, as shown in FIG. 8, thethickness of the silicon oxide film 3 after the mechanical polishingsatisfies the relation B>A, where A is the thickness of the region wherethe heating resistors 10 and the resistance temperature detectors 9 and11 are deposited, and B is the thickness of the region where thesemembers are not deposited. The steps 14 conforming to the patterns ofthe heating resistors 10 and the resistance temperature detectors 9 and11 are retained.

The step reduction may be accompanied by the multilayer structure of thesilicon nitride 4 described in Embodiment 1. In this way, the stress canbe reduced further, and cracks can be effectively suppressed.

High-temperature annealing of the heating resistors 10 and theresistance temperature detectors 9 and 11 with metallic materials mayproduce hillocks 16 (local protuberances), as shown in FIG. 9. Suchhillocks 16 add to the steps 13, and increase the stress. The methodsdescribed in Embodiments 1 and 2 are thus particularly effective againstsuch hillocks 16.

REFERENCE SIGNS LIST

-   1 Silicon substrate-   2 Thermally oxidized film-   3, 5 Silicon oxide film-   4 Silicon nitride film-   6 Diaphragm portion-   7 End portion of etching mask-   8 End portion of diaphragm portion-   9,11 Resistance temperature detectors-   10 Heating resistors-   12 Terminal electrodes-   13,14 Steps-   15 PIQ film-   16 Hillocks

1. A thermal air flow sensor comprising: a semiconductor substrate; aheating resistor, resistance temperature detectors, and an electricalinsulator that includes a silicon oxide film, wherein the heatingresistor, the resistance temperature detectors, and the electricalinsulator are formed on the semiconductor substrate; and the heatingresistor and the resistance temperature detectors being formed on adiaphragm portion of the semiconductor substrate; the silicon oxide filmformed as the electrical insulator being disposed on the heatingresistor and the resistance temperature detectors; the thermal air flowsensor further comprising a silicon nitride film laminated on thesilicon oxide film; wherein steps are arranged on the silicon oxidefilm, and the steps correspond to patterns of the heating resistor andthe resistance temperature detectors; and wherein a thickness of thesilicon oxide film where the heating resistor and the resistancetemperature detectors are disposed is smaller than a thickness of thesilicon oxide film where the heating resistor and the resistancetemperature detectors are not disposed.
 2. The thermal air flow sensoraccording to claim 1, wherein heights of the steps are smaller thanheights of the heating resistor and the resistance temperaturedetectors.
 3. The thermal air flow sensor according to claim 1, whereinthe silicon nitride film comprises plural layers.
 4. The thermal airflow sensor according to claim 1, wherein a hillock is formed on theheating resistor and the resistance temperature detectors.